Solar energy is a promising source of power, but in part due to the expense of manufacturing devices which are able to convert solar energy into electrical energy, the economic feasibility of solar energy is presently limited. Thin films (layered upon one another to form photovoltaic cells) have recently become advantageous, but remain relatively expensive to manufacture due to the complex deposition process used with conventional materials. Additionally, degradation of electrical power output occurs over time in some conventional thin film photovoltaic cells such as amorphous silicon cells.
Amorphous silicon based thin films are made in groups of three films (trios) forming a PIN junction cell and in multiple layers of these trios forming a PIN multijunction cell. Multijunction thin film photovoltaic cells, such as those made of amorphous silicon, have semiconductor alloys of different composition forming each of the thin film trios making up the cell. The films have different compositions to enable each trio of the cell to have a bandgap different from the other trios. The difference in bandgap between trios enables each trio in a cell to absorb a different portion of the light spectrum, (i.e. a different range of photon energies). With conventional materials, however, the bandgap of the material of one film can only be made different from another film by varying the elemental composition of the films.
It is conventional to use silicon based materials to form a multijunction cell made up of three film layers in each PIN junction trio. The same silicon based material makes up each film in the trio and the films are doped to establish their p-type, n-type or intrinsic characteristics. The bandgap of the films within a trio is usually the same, but between trios, the bandgap is usually different for the reasons described above. The bandgap between trios differs by making one trio of a silicon based material with a specific bandgap, (e.g. silicon carbide (SIC)), the next trio of another silicon based material with a different bandgap (e.g. silicon) and the next trio of another silicon based material with a still different bandgap (e.g. silicon germanide (SiGe)). Each trio in this multijunction cell absorbs a different portion of the spectrum of light. Since each trio utilizes the energy of a different portion of the light spectrum, a broader spectrum of light is converted to electrical energy, and therefore efficiency and electrical output are increased in the photovoltaic cell. However, these advantages can only be realized by controlling and changing the chemical composition of the thin films from trio to trio during manufacture which is a technically complex task.
It is technically complex to change the type of thin film material during common manufacturing processes, such as physical vapor deposition, used to make such thin films. This process involves depositing an alloy material onto a substrate while changing dopants for each film within a trio, and then depositing a different alloy to make the films of the next trio. When this change in alloy is necessary, a different source of material must be used, and therefore the deposition chamber must have all of the previously deposited material removed.
In order to avoid the need to use different elemental compositions for each trio of semiconductors making up each PIN junction, it would be desirable to find a material having semiconductor characteristics making it suitable for use in a photovoltaic cell, and also having the capability of having its bandgap widely varied without requiring alloying. However, it is necessary that the material not have other characteristics which would make it unsuitable for use in a photovoltaic cell.
Such a single material used for all layers of a cell would need to have an optical bandgap variable over a wide range. Two films of the same material having different work functions and forming a rectifying contact can form a photovoltaic cell, but to form a multijunction cell, the bandgap generally has to be changed between trios. No material has been found that can be varied this way.
Despite the potential of amorphous, hydrogenated carbon for use in a photovoltaic cell, amorphous, hydrogenated carbon has never been used or suggested as useful as an active element in the voltage generating portion of a photovoltaic cell.
In U.S. Pat. No. 5,206,534 by Birkle et al., amorphous, hydrogenated carbon has been used in a photovoltaic cell in its undoped form. Although this reference teaches to use amorphous carbon in a photovoltaic cell, the amorphous carbon film is passive, i.e. not an active charge carrier generating element. The photoexcited charge carriers are most likely produced in the monocrystalline GaAs or InP. Additionally, the efficiency of the cell is not mentioned.
In "Photoresponse characteristics of n-type tetrahedral amorphous carbon/p-type Si heterojunction diodes" by Veerasamy et al. (Appl. Phys. Lett., Vol. 64, No. 17, Apr. 25, 1994, pp. 2297-2299), the author describes the use of tetrahedral, non-hydrogenated amorphous carbon (also called amorphous diamond and amorphic diamond) as an active element in a photovoltaic cell. However, the tetrahedral amorphous carbon has no hydrogen in it, and this results in chemical and thermodynamic differences between tetrahedral amorphous carbon and amorphous, hydrogenated carbon.
There are problems associated with using amorphous, hydrogenated carbon as anything other than a conductive substrate or container for enclosing the materials which contribute to the current. Problems with amorphous, hydrogenated carbon making it unsuitable for use as an element which contributes to current generated by a photovoltaic cell are seemingly insurmountable, based on the prior art. For example, there are too many defect sites in the amorphous, hydrogenated carbon structure to make it a suitable material for a photovoltaic cell. Additionally, the lifetime of the minority carriers has been found to be too short. Furthermore, carbon has single, double, and triple bonds in its different morphologies which makes it very complicated, and therefore less likely to have the ability to exhibit photovoltaic characteristics. It is less likely that amorphous, hydrogenated carbon would exhibit photovoltaic characteristics due to the type of interatomic bonds, as is illustrated by examining a common photovoltaic material: silicon. The bonding of silicon, which is tetragonal, is critical to the material's ability to exhibit photovoltaic characteristics. Since carbon can have different bonds than silicon, it would seem less likely that amorphous, hydrogenated carbon would exhibit photovoltaic characteristics.
Amorphous, hydrogenated carbon would seem to be desirable for use in active semiconductor devices. However, scientists have attempted to use amorphous, hydrogenated carbon in this capacity and they have found substantial problems with it. However, it would be desirable, if these problems could be overcome, to use amorphous, hydrogenated carbon in a photovoltaic cell due to the unique advantages it possesses.